1. Field of the Invention
The present invention relates to an inkjet printer and inkjet printing method that, while scanning a printing head, ejects printing liquid such as ink on a printing medium to print an image. More particularly, the present invention relates to an inkjet printer and inkjet printing method that enables, in both forward and backward scans of a printing head, the image to be printed.
2. Description of the Related Art
Currently, a serial type inkjet printer is widely used, that is provided with: a carriage mounted with a printing head including a plurality of nozzles that eject ink and an ink tank; a conveyance unit that conveys a printing medium; and a control unit that controls them. The inkjet printer repeatedly performs: a main scan in which, while moving the carriage in a direction (main scanning direction) orthogonal to a conveyance direction of the printing medium, the ink is ejected from the printing head to perform printing; and a sub scan in which, at the time of printing, the printing medium is conveyed by a distance corresponding to the printing width of the printing head. Also, many of the currently used inkjet printers are ones that can use a plurality of color inks to print full color images. For example, printing heads that can eject inks such as yellow (Y), magenta (M), cyan (c), and black (b) inks are mounted on a carriage, and these inks are used to print full color images. In many cases, in the carriage, a plurality of printing heads corresponding to the respective inks are sequentially arranged along the main scanning direction.
In the case where along a forward direction in the main scan, the printing heads are arrayed in the order of Bk, C, M, and Y, a printing order at the time of printing on a printing medium by a forward scan is the order of Bk, C, M, and Y, whereas a printing order at the time of printing by a backward scan is the order of Y, M, C, and Bk. As described, the printing order is different between the forward and backward scans and, therefore, the order to overlap the inks on the printing medium is different between the forward printing and the backward printing. For this reason, the hue is different depending on the conveyance distance of the printing medium, which may cause color unevenness to result in a reduction in image quality.
A document that discloses the technique for preventing color unevenness in so-called bidirectional printing in which the forward printing and backward printing are performed is Japanese Patent Laid-Open No. 2001-180017. Japanese Patent Laid-Open No. 2001-180017 discloses a printing method that counts the number of dots for each ink color on the basis of printing data, and if an ink driving threshold where color unevenness occurs is exceeded, it fixes the printing direction to a forward or backward direction. If the printing direction is fixed to perform printing as described, the printing order of respective inks becomes constant and, therefore, color unevenness can be suppressed.
However, in the technique disclosed in Japanese Patent Laid-Open No. 2001-180017, if the ink driving threshold where color unevenness occurs is exceeded, the printing direction is fixed to the forward or backward direction and, therefore, there arises a problem in that the printing time is increased.